An electric oven is located in an area with ambient temperature of 90 degrees F. The oven contains a 27KW heating element and connected to 240 volts. The conductors are to be copper with type THWN-2 insulation. The heater is expected to operate for more than three hours at a time. What size conductor should be used?
To determine the size of conductor needed, we will need to consider the current (I) and the ambient temperature (Ta).
First, let's calculate the current using the power (P) and voltage (V) of the heating element:
I = P / V
I = 27,000 W / 240 V
I = 112.5 A
Next, let's determine the temperature correction factor for the ambient temperature of 90 degrees F (Ta = 90°F). This correction factor is necessary because the conductivity of copper is affected by temperature. The correction factor can be found in the National Electrical Code (NEC) - Table 310.15(B)(2)(a):
For 90°F, the correction factor (C) for copper conductors with THWN-2 insulation is 0.91.
To calculate the adjusted current, we multiply the current (I) by the correction factor (C):
Adjusted Current (Ia) = I * C
Ia = 112.5 A * 0.91
Ia ≈ 102.38 A (rounded to the nearest hundredth)
Now, let's determine the minimum required size of the conductor using the adjusted current (Ia) and the NEC - Table 310.15(B)(16) for copper conductors:
Based on the table, a 4 AWG conductor (American Wire Gauge) is rated for 100 A.
Therefore, a 4 AWG conductor should be used for this electric oven.